1. The Field of the Invention
This invention relates to signal transmissions, and more particularly to the transmission of optical signals.
2. The Relevant Technology
The quality and performance of a digital fiber optic transmitter is determined by the distance over which the transmitted digital signal can propagate without severe distortions. The bit error rate (BER) of the signal is measured at a receiver after propagation through dispersive fiber and the optical power required to obtain a certain BER, typically 10−12, called the sensitivity, is determined. The difference between the sensitivity at the output of the transmitter and the sensitivity after propagation is called the dispersion penalty. This is typically characterized by the distance over which a dispersion penalty reaches a level of ˜1 dB. A standard 10 Gb/s optical digital transmitter, such as an externally modulated source can transmit up to a distance of ˜50 km in standard single mode fiber at 1550 nm before the dispersion penalty reaches the level of ˜1 dB, called the dispersion limit. The dispersion limit is determined by the fundamental assumption that the digital signal is transform limited, i.e. the signal has no time varying phase across its bits and has a bit period of 100 ps, or 1/(bit rate). Another measure of the quality of a transmitter is the absolute sensitivity after fiber propagation.
Three types of optical transmitters are presently in use in fiber optic systems: (i) directly modulated lasers (DML), (ii) electroabsorption modulated lasers (EML), and (iii) externally modulated Mach Zhender (MZ) lasers. For transmission in standard single mode fiber at 10 Gb/s, and 1550 nm, it has generally been assumed that MZ modulators and EMLs can have the longest reach, typically reaching 80 km. Using a special coding scheme, referred to as phase shaped duobinary, MZ transmitters can reach 200 km. On the other hand, directly modulated lasers (DML) reach <5 km because their inherent time dependent chirp causes severe distortion of the signal after this distance.
In transmitters marketed as the Chirp Managed Laser (CML™) by Finisar Corp. of Sunnyvale Calif., a frequency modulated (FM) source is followed by an Optical Spectrum Reshaper (OSR) which uses the frequency modulation to increase the amplitude modulated signal and partially compensate for dispersion in the transmission fiber. The frequency modulated source may include a directly modulated laser (DML). The Optical OSR, sometimes referred to as a frequency discriminator, can be formed by an appropriate optical element that has a wavelength-dependent transmission function. The OSR can be adapted to convert frequency modulation to amplitude modulation.
Inasmuch as frequency modulation in the output of the laser in such transmitters is converted to amplitude modulation, it is advantageous to use a laser having fast frequency response and high FM efficiency. In some instances, lasers satisfying these criteria have diminished output power. Accordingly, it would be an advancement in the art to provide an optical transmitter including a directly modulated laser and OSR that also provides high output power.